Water Quality
Standards
Summary information extracted from: Guidelines for drinking-water quality, 2nd ed. Addendum to Vol. 1. Recommendations. - Geneva, World Health Organization, 1998. pp. 13-14.
The cyanobacteria, also known as blue-green algae, are a major group of bacteria that occur throughout the world. Freshwater cyanobacteria may accumulate in surface water supplies as "blooms" and may concentrate on the surface as blue-green "scums." Some species of cyanobacteria produce toxins, which are classified, according to their mode of action, as hepatotoxins (e.g. microcystins), neurotoxins (e.g. anatoxins), and skin irritants. The hepatotoxins are produced by various species within the genera Microcystis, Anabaena, Oscillatoria, Nodularia, Nostoc, Cylindrospermopsis, and Umezakia. Most hepatotoxins (all cyclic heptapeptides) are microcystins. The chemical structure of microcystins includes two variable amino acids and an unusual aromatic amino acid, ADDA (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid), containing a substituted phenyldecadienoic acid. Microcystin-LR is a cyclic heptapeptide with a relative molecular mass of about 1000.
The growth of cyanobacteria and the formation of blooms are influenced by physical, chemical, and biological factors such as light intensity, temperature, water turbulence, presence of inorganic nitrogen and phosphorus nutrients, and pH.
The major route of human exposure to cyanobacterial toxins is the consumption of drinking-water. Some people are also exposed to cyanobacterial toxins through the consumption of certain algal food tablets.
Blue-green algae have been known to cause animal and human poisoning in lakes, ponds, and dugouts in various parts of the world for over 100 years. Cyanobacterial toxins have been implicated in human illness following consumption of water from certain municipal supplies in several countries, often after algal blooms had been treated with copper sulfate. In most cases, the cyanobacteria and sometimes the toxins involved have been identified, but the levels of toxin associated with illness have not been established with any certainty.
There are insufficient data to allow a guideline value to be derived for any cyanobacterial toxins other than microcystin-LR. A 13-week study in mice with microcystin-LR is considered the most suitable for the derivation of a guideline value for this toxin. In this study, a NOAEL of 40 µg/kg of body weight per day was determined for liver pathology. A TDI of 0.04 µg/kg of body weight per day can be calculated by applying an uncertainty factor of 1000 (100 for intra- and interspecies variation, 10 for limitations in the database, in particular lack of data on chronic toxicity and carcinogenicity) to the NOAEL. An allocation factor of 0.80 is used for the proportion of daily exposure arising from drinking-water, because there is little exposure from any other source or route. The resulting guideline value for total microcystin-LR (free plus cell-bound) is 1 µg/litre (rounded figure) in drinking-water.
The guideline value thus calculated is supported by a 44-day study in which pigs were exposed, in their drinking-water, to an extract from M. aeruginosa containing microcystin-LR.
The guideline value of 1 µg/litre is provisional as it covers only microcystin-LR, the database is limited, and new data for the toxicity of cyanobacterial toxins are being generated.
